This invention relates to a combined cycle power plant comprising a combined gas turbine system and steam turbine system, and more particularly to a water level control system for controlling a water level in a boiler drum incorporated in the combined cycle power plant.
In a conventional power generation plant, a generator is driven by a steam turbine incorporated in a steam turbine plant or a gas turbine incorporated in a gas turbine plant, but in recent years, a combined cycle power plant in which a steam turbine is driven by waste heat from a gas turbine has been developed. Such combined cycle power plant has been expected in a viewpoint of an improved high heat efficiency in comparison with the ordinary steam power plant generator plant.
FIG. 11 shows a diagrammatic view of a conventional combined cycle power plant, which generally comprises a gas turbine system 1, a waste heat recovery system 2, and a steam turbine system 3.
The exhaust gas EG used for driving a gas turbine 4 of the gas turbine system 1 is supplied to a waste heat recovery boiler 5 as a heat recovery steam generator of the waste heat recovery system 2, and the exhaust gas EG is then cooled by the heat exchanging operation during the passing through a super heater 6, an evaporator 7, and an economizer 8 arranged in this order in the waste heat recovery boiler 5. The heat-exchanged and cooled gas is thereafter exhausted in the atmosphere.
The waste heat recovery boiler 5 of the waste heat recovery system 2 is also equipped with boiler drum 9 which is connected to the evaporator 7 through a circulation pump 10, thus constituting a closed circuit. To the drum 9, a water level detector 9A is operatively connected to detect the water level in the drum 9, and the detector 9A transmits a signal A representing the water level to a water level controller 11, into which are inputted a signal B representing steam flow rate from a steam flow meter 12 and a signal C representing feed water flow rate from a feed water flow meter 13 as well as the signal A. The controller 11 then transmits a signal D in response to these signals A, B and C to a feed water control valve 15 provided for a feed water line 14 to carry out the open-close control of the control valve 15.
The feed water regulated in the flow rate by the feed water control valve 15 is once fed into the economizer 8 to be preheated and then guided into the drum 9. The water stored in the drum 9 is fed into the evaporator 7 through the circulation pump 10 to evaporate the same into the steam, which is again returned to the drum 9. The steam returned to the drum 9 is separated there into gas and liquid components, and the liquid component is again fed into the evaporator 7 and the gas, i.e. steam, is fed into the super heater 6 to obtain a dried steam, which is then fed into a steam turbine 17 through a steam line 16.
The steam fed into the steam turbine 7 drives a generator 18 and the steam utilized for driving the generator 18 is thereafter fed into a condenser 19 in which the used steam is condensed into the condensate. The condensate is fed into the economizer 8 as a feed water by the actuation of a condensate pump 20, while the feed water rate or amount being adjusted by the open-close control of the feed water control valve 15.
The feed water control valve 15 is generally controlled by a system shown in FIG. 12 as a block diagram. Referring to FIG. 12, to the drum water level controller 11 are respectively inputted the signal A from the water level detector 10, the signal B from the steam flow meter 12 and the signal C from the feed water flow meter 13. The steam flow rate signal B and the feed water flow rate signal C among these signals A, B and C are compared in a comparator 21, and the thus obtained comparison signal is differentiated by a differentiator 22 and then transmitted into a PID (proportion integral-differential) controller 23.
In addition to the differentiated signal, the drum water level detection signal A and a drum water level preset signal E are inputted into the PID controller 23, in which these signals are processed and the valve control signal D is then transmitted to the feed water control valve 15.
The open-close operation of the feed water control valve 15 is controlled in response to the valve control signal D from the drum water level controller 11 thereby to control the feed water amount into the economizer 8, and even in case the load of the waste heat recovery system 2 varies at the ordinary operation time of the gas turbine system 1, the water corresponding to this load variation is fed appropriately, so that the water level in the drum 9 can be kept relatively stable.
As described hereinabove, since the water level controller for the boiler drum of the conventional type controls the feed water control valve 15 by the water level controller 11 in response to the signals A, B, and C representing three main functions mentioned hereinbefore, the water level of the drum 9 can be properly controlled during the ordinary operation of the gas turbine system 1 and the waste heat recovery system 2.
However, in case the gas turbine system 1 accidentally fires which may result in the operation stop of the system and the waste gas EG is not fed to the waste heat recovery system 2 as a heat source therefor, the steam passing the evaporator 7 cannot be evaporated, thus damaging the evaporating function, and a rapid phase variation is caused to the evaporated steam. The steam is accordingly condensed and the volume thereof is reduced or shrunk, and this shrinkage of the steam volume causes the water in the drum 9 to flow into the evaporator 7 to adversely extremely lower the water level in the drum 9 or the circulation pump 10 is overloaded thereby to accidentally trip the waste heat recovery system 2.
When the water level in the drum 9 lowers, it is necessary to widely open the feed water control valve, and the condensate of the amount more than the predetermined amount will have to be fed into the drum 9 to supplement the water therein. For this reason, the water level in the condenser 15 extremely lowers, and accordingly, when the combined cycle plant is regenerated, the condensate pump 20 is idly operated to result in a generation of an operational fault such as cavitation.